A frequency converter translates the frequency components of an input signal in the frequency domain so as to translate the input signal from one part of the frequency spectrum to another. In superheterodyne radio and television receivers, a frequency converter is used to change the frequency of the received radio frequency (RF) signal to a fixed intermediate frequency (IF) signal.
A frequency converter comprises an oscillator and a mixer. Typically, the mixer includes a non-linear device such as a diode or a transistor. The oscillator signal and the incoming RF signal are applied as inputs to the mixer. The two input signals beat (or heterodyne) within the mixer to produce a signal which, in addition to having signal components at the frequency of the input signals, has numerous sum and difference frequencies.
The output circuit of the mixer is provided with a tuned circuit for selecting only one beat frequency, i.e., the predetermined IF frequency. By tuning the local oscillator frequency, the output frequency of the mixer is kept constant for all values of received signal frequency.
It is known to use a dual gate FET transistor as a mixer. For example, see Solid State Devices Manual, SC-16, 1975, published by RCA Corporation, Solid State Division. The mixer circuit shown in FIG. 455, page 373 of the above-referenced manual employs a dual gate FET transistor wherein the RF input signal is applied to one gate and the oscillator signal is applied to the other gate. In a similar manner, two individual FET transistors of the same conductivity type with their conduction channels connected in series, may be employed as a mixer. However, such circuit configurations require a substantial number of peripheral components, particulary resistors, for proper operation.
Mixer circuits using complementary symmetry FET transistors have been disclosed in U.S. Pat. Nos. 4,032,851 and 4,090,139, both of which are issued to the present inventor and assigned to RCA Corporation. The mixer circuits shown in the above-referenced patents are of the type in which a separate oscillator would have to be employed. In contrast, circuits of the present invention are embodied in frequency converter circuits, i.e., mixer circuits of the self-oscillating type (also known as an autodyne frequency converter) wherein a separate oscillator is not required.